1. Chapter -8 Digital Signatures
Cryptography Techniques
(CNET-326)

2. Digital Signatures
The most important development from the work on public-key cryptography is the digital signature.
The digital signature provides a set of security capabilities that would be difficult to implement in any other way.
Figure 13.1 is a generic model of the process of making and using digital signatures.
Bob can sign a message using a digital signature generation algorithm. The inputs to the algorithm are the message and Bob’s private key.
Any other user, say Alice, can verify the signature using a verification algorithm, whose inputs are the message, the signature, and Bob’s public key.

3. Digital Signature Model

4. Digital Signature Model
In simplified terms, the essence of the digital signature mechanism is shown in Stallings Figure This repeats the logic shown in Figure On example, using RSA, is available at this book's Web site.
We begin this chapter with an overview of digital signatures. Then, we introduce the Digital Signature Standard (DSS).

5. Digital Signature Properties
The digital signature must have the following
properties:
• It must verify the author and the date and time of the signature.
• It must authenticate the contents at the time of the signature.
• It must be verifiable by third parties, to resolve disputes.

6. Digital Signatures Requirements
On the basis of the properties and attacks just discussed, we can formulate the following requirements for a digital signature.
• The signature must be a bit pattern that depends on the message being signed.
• The signature must use some information unique to the sender to prevent both forgery and denial.
• It must be relatively easy to produce the digital signature.
• It must be relatively easy to recognize and verify the digital signature.
• It must be computationally infeasible to forge a digital signature.
• It must be practical to retain a copy of the digital signature in storage.